The invention relates to a hybrid propulsion system for a vehicle provided with combined mechanical and hydraulic propulsion. The invention is in particular directed to the hydraulic propulsion system of such a vehicle.
For heavy road vehicles, it is known that there is sometimes a desire for providing driving force on several wheel pairs such that the vehicle for example is provided with a driving force on a rear pair of wheels as well as on front pair of wheels. In many cases. It is desirable to be able to control the traction of the vehicle such that one or several wheel pairs may be connected or disconnected from the power source depending on the traction force demand. The propulsion unit may be the same for all the driving wheels or be a combination of different power sources, e.g. a vehicle provided with a mechanical drivetrain connected to an internal combustion engine and a hydraulic power source connected to hydraulic motors. In general, the mechanical drive is used as the main propulsion system for road travel and the hydraulic drive used as an auxiliary drive for rough conditions at low speeds or for a vehicle in a work mode, e.g. when using creep drive for loading or unloading operations. Different examples of such vehicles are for example disclosed in WO 2011/100 206; U.S. Pat. No. 5,361,208; U.S. Pat. No. 3,780,820; EP 505 727; or US 2011/197 574.
Vehicles today are thus provided with traction forces on both rear wheels and front wheels using a combined mechanical and hydraulic drive and are designed and have control systems for enabling, disabling and controlling the different traction systems to be used efficiently. However, there is still a desire to improve the systems in order to improve drivability and provide an efficient hydraulic drive system when using both propulsion systems as well as when using only one of the systems.
It is desirable to provide an efficient hydraulic propulsion system which may be used as an additional propulsion system together with a mechanical propulsion system to power wheels of the vehicle and in an easy way be activated or deactivated depending on the traction demand. In general the hydraulic propulsion system is intended to only be active during slow speeds, for example when there is a desire for additional power or used as the single traction source when performing, work or driving slowly with frequent stop and go. It is therefore a desire to be able to decouple or disconnect the hydraulic drive when using the vehicle at rather high speeds driving on a road only using a mechanical propulsion system. The hydraulic propulsion system should thus be possible to disconnect and connect in an easy way and be able to provide an efficient freewheeling with low frictional losses and low energy consumption during freewheeling, i.e. when the hydraulic propulsion system is not used for providing a traction force to the vehicle. Hence, the invention relates, according to an aspect thereof, to a heavy road vehicle comprising a hybrid propulsion system including at least a first traction wheel, preferably a pair of traction wheels, which forms a first mechanical propulsion system comprising a mechanical drive train including an internal combustion engine provides a traction force to the first traction wheel, or wheel pairs, via a gearbox. The hybrid propulsion system further comprises a second, different traction wheel, or preferably a second pair of traction wheels different from the first wheel pairs, which forms part of a second hydraulic propulsion system comprising a hydraulic pump unit powering at least one hydraulic motor in order to provide a traction force to said second traction wheel or wheel pairs. The hybrid propulsion system also comprises a control unit for control of said second, hydraulic propulsion unit. The control unit may also be connected to and able to control the first, mechanical propulsion system.
According to the present idea is the hydraulic propulsion system of the heavy vehicle connected to a pressure accumulator. The pressure accumulator enables the hydraulic propulsion system to be pressurized without the need to continuously use a hydraulic pump, a feature which may be useful for example during freewheeling of the hydraulically powered wheels.
Hence, the pressure accumulator is connected to the hydraulic motor or motors in order to maintain a pressure in the hydraulic motor. The pressure accumulator may thus be used instead of the pump unit for maintaining the pressure in a closed part of the hydraulic system including the motor or motors instead of providing a continuous flow of hydraulic liquid, by using the pump unit. The use of the pump unit is necessary when there is a desire to provide a continuous flow through the motors, e.g. when using the hydraulic system to provide traction power. The need for a pressurized system when not using the hydraulically powered wheels for traction, e.g., during freewheeling, is to assure there is hydraulic liquid present in the system for lubricating purposes, to be able to quickly switch on the hydraulic drive and the pressure may also be used to keep the hydraulically driven wheels in a freewheeling mode.
The pump may be constructed to be able to provide high pressure flow as well as low pressure flow. The pump unit may be designed to comprise a main pump for delivering a high pressure flow and a charge pump for delivering a low pressure flow. If the main pump is a variable displacement pump it may be able to provide a high pressure flow as well as a low pressure flow. The high pressure flow is used when there is a need or desire for providing a tractive force from the hydraulically driven wheel or wheel pairs. The charge pump is used either for supplying hydraulic liquid to the main pump or for being connected directly to the hydraulic propulsion system and used for providing a low pressure flow through the hydraulic motor, e.g. if there is a desire to provide a cooling flow to the hydraulic motor without any significant traction desired. The charge pump could be of the fixed displacement kind or variable displacement. The hydraulic motors could advantageously be of the fixed displacement kind even though a variable displacement also is possible.
The pressure accumulator may be connected to the pump unit to be pressurized by the pump unit hi order to achieve a high pressure in the pressure accumulator the pressure accumulator is pressurized or loaded by a flow from the main pump of the pump unit.
In order to control the hydraulic propulsion system, it is provided with a number of valves which may stop or control the flow direction of the hydraulic liquid. It is obvious for the skilled person in the art that the valves and hydraulic piping forming the hydraulic system may be designed in an almost endless way of combinations such that the hydraulic propulsion system may be controlled to switch between a first pressure accumulator mode in which the hydraulic motor may be pressurized by the pressure accumulator while there is essentially no flow through the motor and a second pump pressure mode in which there is a flow from the pump unit through the hydraulic motor. In order to work properly, the hydraulic system should be designed such that it is possible to charge the pressure accumulator, preferably from the pump unit used for providing a tractive flow to the hydraulic motor. The pressure accumulator should also be able to be in hydraulic communication with the hydraulic motor when the pump unit is switched off or at idle such that the pressure in the motor is maintained. When the pressure accumulator is used for pressurizing the hydraulic motor it should be possible to cut off the flow through the hydraulic motor in order to maintain the pressure within the motor. There are thus a number of ways to design the system to be able to fulfill these desired criteria which are within the scope of the invention. The hydraulic system may be realized by having a flow directional valve which enables a flow from the pump unit through the hydraulic motor or motors when the hydraulic drive is turned on while enabling the pressure accumulator to be loaded at the same time. If the pressure accumulator is loaded above a certain pressure, the flow to the pressure accumulator may be cut off and the hydraulic drive may continue. When it is desired that the hydraulic drive no longer shall be used, the hydraulic pumps may be turned off and the pressure accumulator and the hydraulic motor (s) may be set in hydraulic connection with each other within a closed part of the hydraulic system such that the pressure in the hydraulic motors is maintained while there is no flow through the system.
The invention further relates to a Method for controlling a propulsion system comprising a first traction wheel forming part of a first propulsion system comprising a mechanical drive train including an internal combustion engine which provides a traction force to said first traction wheel via a gearbox and a second traction wheel forming part of a second propulsion system comprising a hydraulic pump unit for powering a hydraulic motor in order to provide a traction force to said second traction wheel. Said hybrid traction system further comprises a control unit for controlling said second propulsion unit (13). The control unit is programmed to switch of or set said hydraulic pump unit in an idle mode when it is indicated that a hydraulic pressure accumulator connected to the hydraulic propulsion system has a pressure above a defined pressure limit while at the same time there is indicated there is no desire for a flow of hydraulic liquid from the pump unit to the hydraulic motor. The hydraulic propulsion system is thereby controlled such that the pressure accumulator is connected to the hydraulic motor in order to maintain a pressure in the hydraulic motor.
The control system is in particular suitable for a heavy vehicle of a heavy load carrying kind. The system is particularly useful for trucks which in their duty frequently are used in rough conditions, e.g. timber loading trucks which may be used on small provisional roads or tracks in the forest where the path may be loose or muddy and additional traction force is desired. The vehicle may also be another kind of heavy road vehicle which frequently starts and stops during its working time, e.g. a public bus used at least occasionally in city traffic. Hence, the mechanical/hydraulic hybrid drive system is suitable for vehicles to be used as goods or passenger carriers which demands to provide a comfortable and efficient propulsion when traveling at creep speed, e.g. in a frequently “stop and go”-situation as well as when traveling at higher speeds over longer distances. In order to provide the desired traction under circumstances with poor traction conditions shall both systems also be possible to use simultaneously.
A heavy vehicle of the kind described above shall thus preferably be adapted to run smoothly on normal roads at a relatively high velocity, e.g. up to 90 km/h, while also assure traction at low speeds on unpaved, provisional roads. In order to function in a desirable way under different conditions as exemplified above, the complementary drive, i.e. the hydraulically propelled front wheels, shall be able to be disconnected when driving at high speeds and being able to provide an additional traction force when desired, normally at relatively low speeds. In general, there is no need for using the complementary hydraulic propulsion units above 30 km/h even though it may be advantageous to use them sometimes up to 50 km/h. The vehicle may also be provided with a creep drive function at low velocities when only the hydraulic drive is used.
In order to function efficient for the above described conditions it is thus desired to provide a hydraulic propulsion system which in an efficient way is disconnected and while disconnected reduces the energy consumption. This may be achieved by using the pressure accumulator in order to maintain a desired pressure in the hydraulic system without the need for a continuously working pump. In order to be able to turn off the pump for a longer time is the main flow preferably cut off such that a static pressure in the system is maintained. In some cases it may be desired to have a small leakage flow from the system in order to make the system work properly.